1. Field of the Invention
The present invention relates generally to coatings on metal components that are suitable for use in fuel cell devices, and particularly to protective coatings on stainless steel frames for supporting electrolytes in solid oxide fuel cell devices.
2. Technical Background
The use of solid oxide fuel cells has been the subject of considerable amount of research in recent years. The typical components of a solid oxide fuel cell (SOFC) comprise a negatively-charged oxygen-ion conducting electrolyte sandwiched between two electrodes. Electrical current is generated in such cells by oxidation, at the anode, of a fuel material, for example hydrogen, which reacts with oxygen ions conducted through the electrolyte. Oxygen ions are formed by reduction of molecular oxygen at the cathode.
It is known that substrate type solid oxide fuel cells sometimes utilize metal alloys as separators. Such configuration is described, for example, in the article entitled “Electromagnetic properties of a SOFC cathode in contact with a chromium-containing alloy separator”, by Yoshido Matsuzaki and Isami Yasuda, Solid State Ionics 132 (2000) 271-278. This article describes that when chromium-forming alloys are used as separators, the performance of SOFC cathodes rapidly degrades due to “Cr poisoning”.
The article entitled “Dependence of SOFC Cathode Degradation by Chromium-Containing Alloy on Compositions of Electrodes and Electrolytes”, by Yoshido Matsuzaki and Isami Yasuda, Journal of Electrochemical Society, 148 (2) A126-A131 (2001) also describes the problem of “Cr poisoning”. More specifically, the article states that “it should be of concern that chromium oxyhydroxide vapor generated from an oxide scale (Cr2O3), which is formed on the surface of most high temperature resistant alloys, degrades the performance of cathode under polarization.” The article states that the amount of degradation was found to depend on the composition of the electrolyte on which the electrode was prepared. The article then suggests, in order to reduce Cr poisoning, to use La0.6Sr0.4Fe0.8O3 and Ce0.8Sm0.2O1.9 as the electrode and electrolyte respectively.
The article entitled “A comparative investigation of chromium deposition at air electrodes of solid oxide fuel cells”, by S. P. Jiang, et al., Journal of European Ceramic Society (2002) 361-373 also describes the problem of Cr deposition on electrodes. More specifically, the article states that “Deposition process of chromium (Cr) species were investigated for the O2 reduction on (La, Sr)MnO3 (LSM), Pt and (La, Sr) (Co, Fe)O3 (LSCF) electrodes in presence of chromium-forming alloy interconnect at 900° C. under air flow. For the reaction on LSM electrodes, deposition of Cr species preferentially occurred on the zirconia electrolyte surface, forming a distinct deposit ring at the edge of the LSM electrode while at LSCF electrodes, Cr species deposited on the electrode and electrolyte surface, forming isolated Cr particles. In contrast, there was no detectable deposition of Cr species either on the electrode and electrolyte surface for the O2 reduction on Pt electrodes.” That is, this article also suggests that in order to eliminate or reduce problems associated with Cr deposition on electrolyte/electrode surfaces, one is limited to making electrode/electrolyte from specific materials.
It is known that at sufficient temperatures (e.g., about 725° C. and above), yttria stabilized zirconia YSZ (Y2O3-ZrO2) electrolytes exhibit good ionic conductance and very low electronic conductance. U.S. Pat. No. 5,273,837 describes the use of such compositions to form thermal shock resistant solid oxide fuel cells.
US Patent Publication US2002/0102450 describes solid electrolyte fuel cells which include an improved electrode-electrolyte structure. This structure comprises a solid electrolyte sheet incorporating a plurality of positive and negative electrodes, bonded to opposite sides of a thin flexible inorganic electrolyte sheet. One example illustrates that the electrodes do not form continuous layers on electrolyte sheets, but instead define multiple discrete regions or bands. These regions are electronically connected, by means of electrical conductors in contact therewith that extend through vias in electrolyte sheet. The vias are filled with electronically conductive materials (via interconnects).